1. Field of the Invention
The present invention generally relates to an electrostatic chuck for fixing and carrying wafers composed of silicon or the like in a semiconductor production apparatus. More particularly, it relates to an electrostatic chuck comprising a dielectric ceramic used at a high temperature not less than 250.degree. C. in a CVD (chemical vapor deposition) apparatus, a PVD (physical vapor deposition) apparatus, etc. It also relates to an electrostatic chuck comprising a dielectric ceramic used in the temperature range of room temperature to 500.degree. C.
Although these dielectric ceramics themselves are known, it has not been known that dielectric ceramics have the above-mentioned two different application capabilities. These capabilities have been known by the inventors of the present invention. In the disclosure of the present invention hereafter made, the dielectric ceramics used for the former is referred to as a first invention and the dielectric ceramics used for the latter is referred to as a second invention for the sake of explanation.
2. Prior Art
Clamp rings, vacuum chucks and electrostatic chucks have been used to fix and carry silicon wafers in semiconductor production apparatuses. In the case of vacuum chucks, however, since chucking is performed on the finished bottom surface side of each wafer only through the chucking holes of the vacuum chuck, the chucking force is not applied uniformly to the entire surface of the wafer. In addition to this disadvantage, since many semiconductor production processes are done under high vacuum (10.sup.-3 or less), the vacuum chucks being operated by using vacuum suction force are frequently difficult to be used under such high vacuum. Clamp rings have no warp correction capability, causing nonuniform heating for larger wafers. It is therefore said that electrostatic chucks are useful for fixing and carrying silicon wafers in electron-beam drawing, dry-etching, CVD and PVD apparatuses.
This kind of electrostatic chuck has a structure wherein an electrostatic inner electrode is embedded in a dielectric material. The chucking force of such an electrostatic chuck is represented by the formula shown below. EQU F=S/2.times..epsilon..sub.o .times..epsilon..sub.r .times.(V/d).sup.2
wherein F is chucking force, S is an electrostatic electrode area, .epsilon..sub.o is a dielectric constant of vacuum, .epsilon..sub.r is a relative dielectric constant of a dielectric material, V is an applied voltage and d is the thickness of a dielectric layer.
According to the formula, to increase the chucking force, the following methods can be conceived.
(1) To form a dielectric layer comprising a highly dielectric material PA1 (2) To apply high voltage PA1 (3) To decrease the thickness of the dielectric layer PA1 (1) Must have high chucking force. PA1 (2) Must have highly responsive chucking and releasing operations. PA1 (3) Leak current must be small. PA1 (4) Must have sufficient mechanical strength and rigidity to withstand stress during assembly. PA1 (5) Must have sufficient resistance against heat shock. PA1 (6) Must have high heat conductivity. PA1 (7) Must be composed of materials not adversely affecting wafers.
The electrostatic chuck based on the method (1) has already been proposed by the applicants of the present invention by using a ceramics mainly composed of calcium titanate (highly dielectric material) as a dielectric material (Japanese Patent Application Laid-open Publication No. 4-206948). The electrostatic chucks based on the methods (2) and (3) cause dielectric breakdown at the dielectric layer and are not used practically.
Other than the electrostatic chucks based on the above-mentioned methods (1) to (3), an electrostatic chuck has also been proposed which uses a ceramics made by adding transition elements such as titanium to an alumina material and by sintering the material in a reduction atmosphere to decrease the volume resistivity of the ceramics (Japanese Patent Application Laid-open Publication No. 2-22166). This electrostatic chuck uses a ceramics having low volume resistivity as a dielectric layer to generate minute leak current at the time of voltage application, thereby increasing the chucking force thereof (Johnse-Rahbeck's force).
However, the electrostatic chuck, which uses such leak current as disclosed by the above-mentioned Japanese Patent Application Laid-open Publication No. 2-22166, has various problems as described below.
First, this electrostatic chuck is intended to be used in the temperature range of -100.degree. to 150.degree. C. in electron-beam drawing and dry-etching apparatuses, for example. It is therefore not suited for applications at a higher temperature. More particularly, the volume resistivity of the dielectric material of this electrostatic chuck decreases as the temperature rises. As a result, the leak current becomes excessive at a high temperature not less than 250.degree. C. used in deposition apparatuses such as CVD and PVD apparatuses or high-temperature dry-etching apparatuses, thereby breaking the circuits formed on wafers.
Second, since the volume resistivity of the ceramics used for this electrostatic chuck is controlled depending on sintering conditions, it is necessary to obtain a completely uniform atmosphere in the entire furnace. Even a slight nonuniformity in the atmosphere changes the volume resistivity. It is therefore difficult to control the volume resistivity by using this method, and products having uniform quality cannot be obtained by this method. For this reason, this electrostatic chuck is disadvantageous to mass product ion.
Furthermore, since the above-mentioned ceramic is a mixture of alumina and titanium, its chucking force depends on time. It has a serious disadvantage that sufficient chucking force cannot be obtained immediately after voltage is applied. This problem is assumed to be caused by the fact that titanium is not dispersed uniformly and that titania is generated by oxidation of titanium and a material having a significantly different relative dielectric constant (alumina=10, titania=46) is dispersed. The chucking force which depends on time causes the problem of decreasing the wafer processing capability in semiconductor production apparatuses.
On the other hand, in the case of the electrostatic chuck comprising a highly dielectric ceramic as disclosed by Japanese Patent Application Laid-open Publication No. 4-206948, the highly dielectric ceramic is low in mechanical strength and heat shock resistance. The electrostatic chuck is thus impractical in use at a temperature not less than 250.degree. C.
When using an electrostatic chuck at a temperature not less than 250.degree. C. in CVD, PVD and high-temperature etching apparatuses, the electrostatic chuck must have the following characteristics in general.
Regarding the item (1), the contact between the electrostatic chuck and a wafer is enhanced by high chucking force. The temperature of the wafer can be made close to that of the electrostatic chuck, making the temperature distribution on the wafer surface uniform. Uniform films can thus be formed on wafers by CVD and PVD apparatuses, and finer patterns can be obtained by high-temperature etching apparatuses. The characteristic in the item (2) is essential to enhance the through-put of wafers.
Regarding the item (3), when temperature rises, the volume resistivity of a dielectric material decreases as its own characteristic and the leak current in the material increases. If the leak current is excessive, the danger of breaking the patterns formed on wafers is caused. Furthermore, regarding the item (4), when an electrostatic chuck is assembled in an apparatus operating at a high temperature, the electrostatic chuck must be secured mechanically by using screws or the like or joined by heat resistant joining (by brazing, etc.). The electrostatic chuck must thus have sufficient mechanical strength to withstand the stress generated during screw tightening or the stress caused by the difference in thermal expansion during temperature change. Furthermore, the electrostatic chuck must also have high rigidity, since the surface and shape of the electrostatic chuck must be finished highly accurately to allow highly fine patterns to be formed on wafers.
Regarding the item (5), the electrostatic chuck must be superior in heat shock resistance to withstand the thermal strain due to nonuniform temperature distribution inside the electrostatic chuck when the bottom surface of the electrostatic chuck is cooled for temperature control of wafers or when the electrostatic chuck is used at a high temperature. In addition, regarding the item (6), the material of the electrostatic chuck which makes contact with wafers is desired to be superior in heat conductivity. This is because nonuniform temperature distribution caused on the wafer surface during wafer processing makes uniform filming and fine pattern forming very difficult. Moreover, regarding the item (7), the electrostatic chuck must be composed of elements which do not adversely affect silicon wafers to prevent the chuck from deteriorating the characteristics of silicon wafers, since the electrostatic chuck directly contacts wafers.
In spite of these requirements to be satisfied, a conventional electrostatic chuck comprising a ceramics with titanium added to alumina has large leak current and is inferior in responsivity as described above. On the other hand, another conventional electrostatic chuck comprising a highly dielectric ceramics is inferior in mechanical characteristics. Accordingly, these conventional electrostatic chucks cannot satisfy the characteristic requirements in the above-mentioned items (1) to (7).
In the case of electrostatic chucks used in the relatively low temperature range of room temperature to less than 250.degree. C., not used in the above-mentioned high temperature range, an electrostatic chuck composed of synthetic resin has been used conventionally. However, this type of electrostatic chuck has many problems; its service life is short because of its low wear resistance and low heat resistance; it cannot achieve sufficient flatness and pallelism required for processing wafers. Furthermore, the resin is low in dielectric constant, making dielectric polarization difficult. Although high voltage must be applied to obtain practically strong chucking force, since the resin is low in dielectric strength, high voltage cannot be applied to the electrostatic chuck. The obtained chucking force is therefore limited. For these reasons, the electrostatic chuck composed of synthetic resin is not yet practical. Besides, the electrostatic chuck cannot be used at a temperature not less than 50.degree. C. because of the low heat resistance of the resin.
To overcome these difficulties, instead of using the electrostatic chucks composed of synthetic resin, electrostatic chucks composed of polycrystalline ceramics superior in corrosion resistance, wear resistance and accuracy have been used these days. As this polycrystalline ceramic material, a ceramics including Al.sub.2 O.sub.3 as a main ingredient and TiO.sub.2 (Japanese Patent Application Laid-open Publication No. 62-264638) and highly dielectric ceramics such as CaTiO.sub.3 and BaTiO.sub.3 (Japanese Patent Application Laid-open Publication No. 2-339325) proposed by the applicants of the present invention have been examined.
The electrostatic chuck composed of alumina ceramics including the above-mentioned TiO.sub.2 has a volume resistivity value of about 10.sup.11 .OMEGA.cm and its chucking force is obtained by minute leak current. In actual practice, however, a few mA of leak current flows and the electrostatic chuck has the danger of adversely affecting the circuits formed on silicon wafers.
In addition, the electrostatic chuck composed of highly dielectric ceramics is relatively low in mechanical strength and its chucking characteristic is not constant when used in a reduction atmosphere. The electrostatic chuck must thus be used in limited environments.